The carcinogenic potency of tritium has been documented in newborn mice following administration of tritiated thymidine at various dose levels. Our previous studies showed that this particular compound, administered in the dose range of 0.3 to 1.5 micron Ci per gram was carcinogenic in newborn C57B1 mice. No data exist on a possible carcinogenic potency of other tritiated precursors nor of tritiated water. No data are available for the possible long term toxicity of tritiated water. The need for additional data is obvious in view of the release of tritiated water from nuclear power reactors and nuclear fuel reprocessing plants which will be in operation in the future. Our current research showed that tritium from tritiated water is partially retained in the organic component of all organs tested in newborn, juvenile as well as adult mice. The relative specific activity (R.S.A.), that is, the amount of tritium retained from interstitial water at any time, per hydrogen atom respectively present in water and in the organic component of tissue, is time dependent and increases by a factor of about 20 within a time interval of 28 days, whereas the absolute activity decreases by a factor of one thousand. R.S.A. figures for all precursors tested are also time dependent but vary widely in pattern and in value.